Effect of friction, temperature and velocity on finite element predictions of metal flow lines in forging

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Effect
of friction, temperature and velocity on finite element predictions of metal
flow lines in forging

In metal forming, especially
in forging, metal flow lines formed during rolling or drawing play decisive
role at product strength and integrity [1-4]. Of course, they vary globally
with plastic deformation due to metal forming. Therefore, they store the
history of the metal formed products and they are being utilized for qualitatively
evaluating product quality or durability of power transmission or load
supporting parts particularly including bearings and gears.

The major factors affecting
the predicted metal flow lines can be empirically summarized as developer-side
factors including finite element technique employed and algorithm of metal flow
line prediction and user-side factors including process conditions and material
properties. From the standpoint of users, process conditions including
friction, temperature and velocity and material properties including flow
stress and thermal properties are most important. In spite of its significance,
their effects on the metal flow lines have been sufficiently studied. In this article,
a three-stage hot forging process is studied to reveal the effect of process
conditions and material properties on metal flow lines and draw a guideline
for flow stress at the small strain and strain rate.

Description
of process under investigation:

Figure 1 shows a three-stage
hot forging process to fabricate initial blank of a ring rolling process for
outer race of a bearing. Its process conditions and analysis information are as
follows: The material is STB2 of which flow stress is expressed as in Figure 2.
It is assumed that the initial temperature is uniform as 1130℃. The dwelling time before
upsetting is 0.5 seconds. All the thermal information is summarized in Ref.
[5].

To investigate into the
effects of temperature on metal flow lines, isothermal and non-isothermal
analyses were conducted using the flow stress depicted in Figure 3. The initial
temperature of material is assumed 1130℃. The
coefficient of Coulomb friction was assumed 0.3 and the punch speed 200 mm/s.

Figure 3. Comparison of isothermal and non-isothermal simulations

Figure 3(a) and Figure 3(b)
show the predictions obtained by the isothermal and non-isothermal analyses,
revealing that the temperature is not a major factor. Isothermal analyses were
conducted to investigate into effects of friction on metal flow lines, under
various frictional coefficients of 0.1, 0.2, 0.3 and 0.4. For these analyses,
it was assumed that temperature, initial strain and velocity were 1130 ℃, 0.0001 and 200mm/s,
respectively.

Figure 4. Maximum radius with frictional coefficients

The predicted maximum radii of
the material at the final stroke are listed in Figure 4, indicating that there
is a distinct difference between frictional coefficient of 0.2 and the others.
However, frictional coefficients ranging from 0.2 to 0.4 cause negligible
difference, implying that frictional condition is not a major factor if it is
normal.

Isothermal analyses are
conducted under various velocities to reveal their effects on metal flow lines.
For these analyses, it was assumed that temperature, initial strain and
coefficient of Coulomb friction were 1130℃,
0.0001 and 0.3, respectively. A mechanical press of which stroke and speed are
220mm and 90 rpm, respectively and hydraulic press with constant velocities
200mm/s and 400 mm/s are considered.

Figure 5 compares the metal
flow lines obtained under the velocity conditions tested. It is very difficult
to discriminate three sets of predicted metal flow lines in Figure 5, implying
that the velocity is not a major factor.

The predictions of Figure 5(c)
were selected to be compared with the experiments, as shown in Figure 6. The
comparison shows that the predicted metal flow lines are relatively smooth
especially near the maximally barreled region. It is believed that relatively
small predicted dead metal region near the central axis may cause the
difference. This metal flow difference in upsetting causes the difference shown
in Figure 7 between the predictions and experiments in the piercing stage.

Figure
7. Comparison of experimental and predicted metal flow lines at the second
stage when the same conditions as Figure 4(a) are employed

In
the article, to be posted next week, the influence of flow stress on the FE
prediction of metal flow lines will be presented.